When a function returns a queryset that is only annotated for a filter

[delfick]

Hi,

So in making my QuerySets defined as generic so that django-stubs starts to understand .annotate properly I have a problem where I don't want to always recognise that a queryset is annotated.

For example

def make_my_filtered_queryset(qs: MyQuerySet) -> MyQuerySet:
    return qs.annotate(some_count=Count(....)).filter(some_count==1)

After I make my queryset generic it starts to complain at me along the lines of

Incompatible return value type (got "MyQuerySet[MyModel@AnnotatedWith[TypedDict({'some_count': Any})], MyModel@AnnotatedWith[TypedDict({'some_count': Any})]]", expected "MyQuerySet[MyModel]")  [return-value]

I'd rather not add several hundred TypedDicts for annotations that aren't relevant out of the immediate .filter that uses them.

I'm wondering what y'all think about this situation and whether it's reasonable for mypy to say an annotated queryset is a superset of it's unannotated equivalent?

[lazizbekravshanov]

There's no built-in "annotated is a subtype of unannotated" relation — the annotation is tracked in the generic parameter and treated invariantly, so MyQuerySet[Model, AnnotatedWith[...]] won't unify with the declared return type MyQuerySet[MyModel]. At the point where the extra column stops being relevant, drop it with an explicit cast:

from typing import cast

def make_my_filtered_queryset(qs: MyQuerySet) -> MyQuerySet:
    annotated = qs.annotate(some_count=Count(...)).filter(some_count=1)
    return cast("MyQuerySet[MyModel]", annotated)

That keeps the annotation visible inside the function (so .filter(some_count=...) type-checks) while presenting the plain queryset type to callers — no need to declare throwaway TypedDicts for annotations that never escape the function. Pragmatically that's what cast is for: narrowing back to the public type at the API edge where the annotation is an implementation detail.

[delfick]

tbf, the annotations get translated at django-stubs mypy plugin time such that they get translated into a kind of type heirarchy.

I think why mypy isn't letting me do what I want is that mypy recognises QuerySet as a mutable container and so the type needs to include all attributes, otherwise it's statically possible for somewhere else to add something to the queryset that doesn't have all the attributes it expects.

In either case, I ended up making #3382 and solving the issue with delfick/extended-mypy-django-plugin#72 in my own extension of the plugin.

Also, specifically to the codebase I work on I've banned typing.cast because it kept getting used to avoid the effort required to make the cast not necessary.